Lasers are presently employed for a wide variety of applications. For example, lasers are employed to process materials, such as by cutting, welding, heat treating, drilling, trimming and coating materials, stripping paint, removing coatings, cleaning surfaces, and providing laser markings. Lasers are also used in many medical applications for precision surgery. Additionally, lasers are used in military applications, including laser weapon and laser ranging systems. Laser communication systems have also been developed in which laser signals are transmitted in a predetermined format to transmit data.
Along with the ever increasing number of applications in which lasers are used, the demands on the laser systems are also ever increasing. For example, a number of applications, including military, materials processing, medical, and communications applications, demand pulsed and continuous wave lasers which emit increasingly higher power levels. In addition, a number of applications demand that the laser system produce an output beam which is of high quality, such as by being diffraction limited and/or exhibiting predominantly or entirely fundamental or TEM00 mode characteristics. Accordingly, the output beam can be more definitely focused to achieve higher brightness. At the same time, many applications require that the laser system produce an output beam which is adaptable or dynamically controllable.
One example of the need for high power, high quality laser beams is illustrated by the laser devices used to focus on remote targets. In these applications, it is advantageous for the laser beam to achieve a maximum brightness at the location of the target. For example, in military applications, it is advantageous to generate a laser beam that is focused on the remote target with maximum intensity. Similarly, in medical applications, it is essential that the laser beam be focused on the target tissue such that surrounding tissue is not affected.
Several different types of laser devices that generate high power laser beams have been developed by The Boeing Company, assignee of the present application. Examples of these laser device are discussed in detail in U.S. Pat. No. 5,694,408 to Bott et al. and U.S. Pat. No. 5,832,006 to Rice et al., the contents of which are incorporated herein by reference.
The basic approach of these laser devices is to amplify a coherent signal emitted from a master oscillator using a phased array of fiber optic amplifiers. A sample of the output optical signal is extracted for comparison to a reference laser beam that has also typically been output by the master oscillator. The sample of the output optical signal and the reference signal are combined by interference, and the interference signal is sampled by an array of detectors. The difference in phase between the sample of the output optical signal and the reference signal is recorded by the detector, and is used as feedback for altering the phase of the output optical signal via an array of phase modulators that are in optical communication with respective fiber optic amplifiers.
In one example, it may be desired that the plurality of output optical signals be capable of being combined into a diffraction limited signal, thereby requiring that the output optical signals emitted by the fiber optic amplifiers have a constant phasefront. Alternatively, the output optical signals emitted by the fiber optic amplifiers may desirably be shaped, steered or tilted in another predefined manner.
To provide the desired phasefront, the laser devices described by U.S. Pat. Nos. 5,694,408 and 5,832,006 have a feedback loop and an array of phase modulators that control the phase modulation of the output laser beam. Specifically, as discussed, a portion of the output laser beam is combined through interference with a reference signal to determine the phase difference for the signals emitted by each fiber optic amplifier. By use of the feedback signal representative of the phase of the output laser beam and knowledge of the desired wavefront, the output laser beam can be generally controlled via the array of phase modulators to produce the desired wavefront and/or to appropriately steer or tilt the wavefront.
Although these laser systems, for the most part, provide reliable and accurate control of the output laser beam, U.S. Pat. No. 6,233,085 to Bartley C. Johnson, the contents of which are also incorporated by reference herein, describes the feedback loop and the associated array of phase modulators in more detail. In this regard, the control methodology described by U.S. Pat. No. 6,233,085 patent can provide for a wide range of phase modulation by avoiding saturation and uncontrolled modulation changes in the output signal.
Once the output optical signals have been emitted by the laser device, atmospheric turbulence or other perturbations may undesirably alter the phase and/or amplitude of the optical signals prior to reaching the target. Thus, even if the laser device is controlled so as to emit optical signals having the desired amplitude and phasefront, the optical signals that are incident upon the target may not have the desired amplitude as a result of atmospheric turbulence. The spatial distribution of the phase and amplitude variations induced by atmospheric turbulence fluctuate on the order of milliseconds. Additionally, the atmospheric turbulence may create regions within the wavefront having zero intensity, i.e., intensity nulls. At such points the phase of the wave is undefined and thus following mathematical terminology are commonly referred to as branch points.
In an effort to address the effects of atmospheric turbulence, some laser devices alter the phases of the output optical signals in a predefined manner to compensate for the anticipated atmospheric turbulence. In this regard, the anticipated influences of the atmospheric turbulence on the signal may be estimated by transmitting a reference beam to the target and then analyzing the spatial variations of the phase of the signal after its reflection by the target and propagation back to the sensor. Optimal efficiency in achieving a tightly focused beam at a target is achieved when the complex amplitude of the outgoing signal is conjugate to that of the reflected reference signal (same intensity and opposite phase). By not adapting the spatial distribution of the phase of the transmitted signal to match that of the reflected reference signal, Strehl may reduced to 10% or even less than 1% depending on the strength of atmospheric turbulence and other optical aberrations and on the size of the aperture of the system. Not adapting the spatial distribution of the amplitude of the transmitted signal to match that of the reflected reference signal, may cause an additional 10-15% decrease in Strehl. Strehl is a metric of the peak intensity of the transmitted beam at the target relative to the peak that would occur for a diffraction limited beam. Futhermore, scintillation and associated branch points can have a profound reduction in the accuracy with which a wavefront sensor measures the wavefront phase resulting in severe drops in Strehl.